In order to obtain high quality paper, it is necessary that the surface of the paper is smooth and substantially free of indentations or valleys. Smooth papers are a prerequisite for good images printed thereon and also for good transfer of ink to paper. Smooth papers are obtained by coating the raw paper surface with a pigment composition. The coating composition to effect this is an aqueous dispersion comprising mainly of mineral pigments such as clay, calcium carbonate, or titanium oxide, and pigment binders of natural protein, for example, casein or soy protein, starch, or synthetic polymer emulsions. Coating compositions are usually applied to a continuous web of material by high speed coating machines, such as blade coaters, air knife coaters, rod coaters and roll coaters. The flow properties of coating color compositions for paper and boards are of significant importance with regard to the runnability (or flow) of the color during the coating operation. These flow properties are often controlled by a "thickener" or "co-binder", which terms are taken to be synonymous in the industry.
In preparing the coating color, the thickener is mixed with the pigment slurry. This may result in what is known in the industry as "pigment shock", due to a strong transient adsorption of the thickener onto the pigment. This causes a rapid increase in viscosity during the early stages of thickener addition. This pigment shock may result in flocculation of the pigment, pigment agglomeration, difficulty in mixing due to momentary solidification of such a pigment slurry, and in severe cases, complete coagulation. Industrial practice cannot tolerate such a phenomenon. Furthermore, when this occurs, the thickener is rendered partially inactive, resulting in less water retention and unsatisfactory rheology before, under, and after the blade. As a consequence, corrective actions during the coating operation are often necessary.
U.S. Pat. No. 4,879,336 discloses an approach to solving the above mentioned problem of pigment shock by mixing clay slurries with a butadiene styrene latex composition when certain poly(vinyl alcohol) copolymers are present in the latex. Another approach to the problem is mentioned in U.S. Pat. No. 3,558,543 that discloses a method of reducing initial thickening (pigment shock) of paper coating when a clay or pigment slurry is mixed with an adhesive solution. This patent uses polyvinyl pyrrolidine mixed with poly(vinyl alcohol) adhesive solutions to eliminate the pigment shock. This reference also discloses that the severity of pigment shock is particularly pronounced when the adhesive is protein material or poly(vinyl alcohol) (see column 1, line 61-63.
U.S. Pat. No. 4,994,112 discloses a paper coating composition containing a water soluble hydrophobically modified hydroxypropylcellulose thickener that has been modified with an alkyl or aralkyl group having preferably 12 to 16 carbons atoms. This thickener enables the paper coating to be applied uniformly at high machine speeds. Another approach to providing a paper coating composition is disclosed in U.S. Pat. No. 5,080,717 which discloses an aqueous paper coating composition comprising clay, a latex, and a thickener of a multi polysaccharide suspension of a hydrophobically modified alkyl hydroxyalkyl cellulose suspended in a low molecular weight polysaccharide and a salt.
None of this prior art discloses nor suggests the instant invention.